BioCycle June 2005, Vol. 46, No. 6, p. 30
Crop trials at The Rodale Institute indicate that further scientific research is needed to enhance understanding of biological cropping system improvement strategies.
Matthew Ryan, David Wilson, Paul Hepperly, James Travis, N. Halbrendt and Alice Wise

IN 2003, a project was initiated at The Rodale Institute (TRI) in Kutztown, Pennsylvania to evaluate the use of compost tea in agricultural production systems. Field trials in 2003 and 2004 measured application effects on potato, grape, and pumpkin crops.
While compost tea has been known and used for centuries, it was not until the late 1990s that interest became more widespread within mainstream scientific and agricultural communities. The popularity was based in part on new and improved methods of making the product, including mechanized brewers with aeration and agitation. Traditional methods simply “brewed” the tea by steeping a compost-filled burlap bag in a bucket of water.
Aerated compost tea brewers now allow the teas to be made with supplemental nutrients such as fish hydrolysate, molasses, humic acid and kelp. They also make production more practical, less messy, and labor-intensive, eliminating offensive odors and requiring as little as 24 hours of brew time per batch.
Another driving force behind the increasing popularity of aerated compost tea was a renewed excitement about its potential to suppress plant diseases and stimulate plant health. With growing interest in and need for nonchemical and organic approaches to pest management, compost tea seemed like the right solution at the right time.
GOING BEYOND TESTIMONIALS
Despite a large body of testimonial and anecdotal support for compost tea, peer-reviewed, scientific information on its disease suppressive effects is more limited. Prior to 2003, The Rodale Institute had not used compost tea on production fields, instead managing crops with fundamental organic farming practices including: Use of disease-resistant crops and varieties; Development of diverse crop rotations incorporating cover crops and, most importantly; Direct application of compost to the soil.
TRI’s work on compost tea was supported by a grant from the Northeast Sustainable Agriculture Research and Education (NESARE) program. Field tests examining the efficacy of compost tea on potatoes, grapes, and pumpkins were conducted in the 2003 and 2004 growing seasons. Grapes were chosen for study because much of the existing compost tea research literature cites positive results for grapes, particularly in controlling powdery mildew. Similarly, potatoes were chosen because large-scale potato growers were reportedly using compost tea practices with promising results. Pumpkins were selected because pumpkin fields frequently suffer powdery mildew problems.
GENERAL METHODS FOR TESTING CROPS
The pumpkin and potato field trials were established at the 333-acre certified organic farm, located near Kutztown, Pennsylvania. Experiments consisted of three primary treatments: 1) Nutrient-supplemented aerated compost tea spray; 2) Nutrient supplement solution spray, and 3) Untreated control (no spray of any kind). The nutrient solution treatment was included to determine if any observed effects arose from nutrient additives (molasses, kelp, etc.) alone, rather than from the compost tea.
Our compost tea recipe was provided by Paul Wagner of the Soil Foodweb, Inc., laboratory in New York, who also collaborated closely on the project by analyzing compost tea, soil, and leaf samples for microbiological properties. The brewers used in our trials were 22-gallon commercial models utilizing a water pump and nozzle system to agitate and aerate the tea.
Immediately after planting potatoes, a soil drench of undiluted compost tea was applied to the plots. Undiluted, aerated compost tea was then applied to the potatoes on a weekly basis throughout the growing seasons. One of the potato trial objectives was to evaluate the effects on late blight, and in 2004, late blight swept swiftly across the Northeast and hit our plots hard. Dr. Paul Hepperly, plant pathologist and TRI Research Manager, assayed disease severity. The data showed that compost tea did not suppress late blight, with the potato foliage quickly succumbing to the disease. On the other hand, the compost tea did improve the quality and consistency of the potato crops, increasing both size and uniformity of the tubers. Similarly, the late blight did not cause any loss in tuber quality, even under prolonged storage, indicating that the pathogen did not infect tubers under our field conditions.
To conduct grape trials, we collaborated with three regional vineyards, a strategy that enabled us to evaluate growers’ adoption of compost tea. Phil Roth of Roth Vineyard in Fairfield, Pennsylvania, has extensive experience using composts and a longstanding interest in sustainable methods. Roth produces his own top-quality compost and makes regular applications in his vineyard, home to some of the oldest Chardonnay vines in Pennsylvania. Barbara Shinn and David Page of Shinn Estate Vineyard in Mattituck, New York had produced and used compost tea in their vineyard for a few years. The third grower-collaborator, Steve Wright of Wright Wine Works in Barto, Pennsylvania, had no previous experience using compost tea.
James Travis, Penn State fruit pathologist and extension agent, assisted with experimental design and disease monitoring in the Pennsylvania vineyards. Also working on the project was Alice Wise, a Cornell Cooperative Extension viticulture expert based near the Shinn Estate Vineyard on Long Island. Both Wise and Travis had previous experience evaluating compost tea use in vineyards, so their participation was a great asset, helping to refine our experimental design, data collection, and analysis.
Collaborating grape growers were supplied with brewers and sprayers, if they did not already have them, and each grower prepared and applied their compost tea independently. Tea was sprayed on a weekly basis with an air-blast sprayer, starting at bud break. In two years of field trials at three different vineyards, we found that compost tea did decrease severity of powdery mildew in some cases. In the first year, compost tea-treated plots had about half the powdery mildew of nontreated plots at two of the vineyards. Other grape diseases were not suppressed, however, and in all trials traditional fungicides were eventually applied to all plots to control disease outbreaks above an acceptable threshold.
In 2003, we did not see any suppression of pumpkin powdery mildew (caused by a different species of fungi than grape powdery mildew), nor did we observe any increase in yields from weekly applications of undiluted aerated compost tea. In 2004, we expanded our experiment to include two additional biologically based disease suppression materials: 1) Serenade, a commercial biofungicide approved for use on organic farms (its active ingredient is Bacillus subtilis, a large, rod-shaped beneficial bacterium), and 2) Reconstituted nonfat dry milk. In other research, both of these materials have been shown to protect crops against powdery mildews.
Our 2004 results showed that compost tea did provide significant early season pow-dery mildew suppression, but when all three materials were combined, we reached much higher levels of control. Pumpkin plots that received all three materials were the least affected by powdery mildew. Based on these results, we believe that combinations of different biologically based materials have potential for controlling powdery mildew in pumpkins and merit further investigation.
CONTROLLING POTENTIAL PATHOGENS IN COMPOST TEA BREWS
To people unfamiliar with the composting process, the idea of mixing compost with water and spreading it on crops may sound like a human disease problem waiting to happen. However, based on our research and experience, human pathogens are only found in manure-based compost tea when molasses is added to the brewing mixture.
In the second year of the study, we eliminated molasses from our compost tea recipe for this reason. As in year 1, numerous tea samples were tested for E. coli by the USDA Environmental Microbial Safety Lab. Eliminating molasses resulted in E. coli levels below detection limits during the second year. Cornell graduate student Allison Horner has also tested the effects of molasses on E. coli growth in aerated compost tea as part of a study funded by the Organic Farming Research Foundation. Her results showed that although E. coli can survive in aerated compost tea, their populations do not increase over a 36-hour brewing period unless molasses is added. Based on these two experiments, we feel that molasses should not be included in any compost tea recipe because it appears to promote E. coli growth. In addition, molasses may reduce the microbiological diversity of compost tea by promoting the growth of more competitive microorganisms.
After working intensively with compost tea for the past two years, we believe it has potential to become a useful component of an integrated crop improvement program. Its benefits are inconsistent, however, and it is certainly no panacea. Compost tea requires further scientific research to clarify its potential benefits and to improve understanding of biological cropping system improvement strategies. As Dr. James Travis put it, “we spent the last 25 years making synthetic methods work. What if we spent 25 years making organic methods work? Just think about what we could do.”
Matthew Ryan, David Wilson and Paul Hepperly are with The Rodale Institute in Kutztown, PA; James Travis and N. Halbrendt are at the Pennsylvania State Fruit Research Center in Biglerville, PA; and Alice Wise is with Cornell Cooperative Extension in Riverhead, New York.